This invention relates methods and apparatus for regulating traffic among contending nodes, being particularly advantageous in a wireless mesh packet radio network system.
In a meshed communication system, packets will favor routes which historically have provided the best performance. As traffic increases, previously acceptable paths will provide degraded performance because of congestion unless alternate, normally slower paths are used or unless communication protocols provide dynamic relief at the affected nodes by changing their characteristics to be more efficient under load.
Meshed packet networks are one of several types of data communication network architectures that support packet communication. Other major types are star (e.g., cellular or 10-base T) and bus (e.g., computer backplane). Mesh networks have several advantages over other architectures for providing high-capacity, high reliability data communication over a large area and to a large number of users.
In a radio-based packet mesh network, an interconnected mesh of data packet sending and receiving nodes collectively captures, routes and delivers data packets in a shared medium. The sharing of this medium results in mutual interference and loss of some packets due to collisions caused by congestion. When a packet is lost, it must be retransmitted, which causes further congestion in the network and causes further packet loss. The packet loss and retransmission consume the limited bandwidth that is used to provide the communication services the network was installed for. It is desirable to minimize this loss of packets so that the network can provide a greater level of performance to the greatest number of users of the network.
Any transmissions or packets that are sent that do not deliver data to the users of the network decrease the efficiency of the network. The higher the efficiency of the network, the more useful work it is performing and the higher its intrinsic value. It is desirable therefore to minimize the overhead packets and transmissions that the protocol uses to communicate to other nodes and to maximize the number of packets that actually deliver data to the users of the network.
As a network becomes more congested and it attempts to deliver more packets than it is capable of, it is desirous that a user get their fair share of the resources. In typical packet networks some users will be favored over others because of the topology of the network or the time distribution of the traffic that they are sending. The network protocols should be designed so that the bandwidth is allocated in a fair and equitable manner regardless of these circumstances.
In a network with intelligent nodes, each node works to manage the traffic through itself. There are several known methods by which this might be done, each has disadvantages, as described below, over the invention described in this patent:
Polling: The controlling or contended node may poll each of the nodes contending for services. The controlling node has imperfect information regarding the servicing needs of the contending (or slave) nodes. Bandwidth used by the controlling node determining if demand exists is overhead which should be minimized. Particularly in a meshed radio network, the extra transmissions can cause further degradation of services because they can increase congestion for multiple nodes.
CSMA/CD: With the Carrier Sense Multiple Access/Collision Detect protocol each node contending for the services transmits (polls) and awaits a response. If the expected acknowledgment is not forthcoming, each of the contending nodes “backs off” or delays an algorithmic amount of time and then retries the poll transmission. This technique is commonly used in wired LAN topologies where all nodes are in reliable communication with each other and thus can reliably hear the acknowledgment to the poll and know when not to transmit. Unfortunately, in many topologies, because of unreliable communication channels, each node has only imperfect information about the state of the targeted node. In this case, only the successful contender (if there was one) is guaranteed to know exactly when the contended node will be free to receive another packet (after the successful node has finished transmitting its packet.)
CDMA: With Code Division Multiple Access, contending nodes transmit by means of a limited set of orthogonal codes. These codes can be selectively detected by decoding each transmission with its own coding sequence. This technique can be used for sending packets from multiple simultaneously transmitting mobile units on the same frequency channel where relative timing can be maintained. However, the frequency channel bandwidth must be increased to handle the additional transmissions. The limitations of this technique are manifold: the processing gain of the coding used limits the number of simultaneously transmitting mobile units. For greatest capacity, the power level of the mobile units must be controlled to be nearly uniform when received at each multiple-station receiving node such as a mobile telephone cellsite. This requires additional protocol overhead which reduces the efficiency of the network.
It is well known that CSMA/CD does not work efficiently as a congestion-limiting scheme for meshed radio networks because of the nature of radio where all nodes cannot ‘see’, or simultaneously communicate with, each other and thus are not able to reliably avoid burst transmissions which block each other. This is a particularly severe problem when the applied load of traffic is large relative to congested node capacity.
Other, more sophisticated protocols (such as GAMA-PS as described by Andrew Muir and J. J. Garcia-Luna-Aceves, “An Efficient Packet Sensing MAC Protocol for Wireless Networks”, MONET 3(2):221–234 (1998)) work better as a congestion-limiting scheme for radio based systems communicating with each other on a single frequency channel (analogous to a wire.) They are; however, unable to handle multiple channels and thus take advantage of the inherent efficiencies available in a meshed network where multiple packets can be sent between different pairs of nodes simultaneously. Protocols designed to handle multiple channels, such as those used for optical networks, have not been designed to efficiently handle unreliable channels, such as those typical in radio networks. Other protocols (such as PRMA as described by D. J. Goodman, R. A. Valenzuela, K. T. Gayliard and B. Ramamurthy, “Packet Reservation Multiple Access for Local Wireless Communications,” IEEE Transactions on Communications, (August 1989) require even more complicated collision detection that is not cost effective or is not available with current radio technology.
The following patents and publications provide further background information:
U.S. Pat. No. 5,384,777 Ahmadi, et. al. Jan. 24, 1995, entitled “Adaptive Medium Access Control Scheme for Wireless LAN”; Ahmadi, Hamid; Bantz, David F.; Bauchot, Frederic J.; Krishna, Arvind; La Maire, Richard O.; Natarajan, Kadathur S.; assigned to IBM Corporation filed Apr. 19, 1993. It discloses an evidently inflexible fixed slot master (base)/slave contention reduction scheme. Access is random access, but there is no teaching of mini-slot categories.
ANSI/IEEE Standard 802.11, 1999 Edition. “IEEE Standards for information technology; Telecommunications and information exchange between systems; Local and metropolitan area networks; specific requirements; Part II: Wireless LAN Medium Access Control (MAC) and Physical Specifications. It teaches a time-based scheme dependent on a single base-station with which all nodes must be in contact.
U.S. Pat. No. 5,471,469: Nov. 28, 1995, entitled. “Method of resolving media contention in radio communication links”; George Flammer and Brett Galloway, assigned to Metricom of Los Gatos, Calif. This disclosure teaches a novel way of reducing contention in a frequency hopped packet radio network but under load (heavy contention) is an inefficient and unfair protocol.
U.S. Pat. No. 5,297,144: “Reservation-based polling protocol for a wireless data communications network”; Gilbert; Sheldon L., Heide; Carolyn L., Director; Dennis L., assigned to Spectrix Corporation of Evanston, Ill. This teaches an inefficient polling mechanism that does not take advantage of the broadcast nature of wireless and requires multiple handshakes between each data transfer.
U.S. Pat. No. 5,818,828: Oct. 6, 1998 entitled “Hybrid multiple access protocol for wireless frequency hopping microcells with adaptive backhaul and heartbeat”; Packer; Robert L., Xu; Milton Y., Bettendorff; John, assigned to Metricom, Inc., Los Gatos, Calif. This disclosure teaches a polling system that requires a Master/Slave relationship to be set up and is inefficient in requiring a poll for every data packet sent and a poll to determine if there is any data available to send.
What is needed is an improvement in communication protocols for mesh networks with multiple channels that can perform efficiently with imperfect channels and provide increased throughput and fair allocation of resources, even under load, with minimal increase in control overhead.